Remove Oxford comma (#2245)

lib/custom_errors.dart

@@ -154,7 +154,7 @@ String list2FriendlyString(List<String> list) {
               (e.key == list.length - 1
                   ? ''
                   : e.key == list.length - 2
-                      ? ', and '
+                      ? ' and '
                       : ', '))
           .join('');
 }